Cast cobalt-base eutectic type alloy

ABSTRACT

A COBALT-BASE ALLOY CONTAINING CHROMIUM, CARBON, ALUMINUM AND SCANDIUM OR YTTRIUM IS UNIDIRECTIONALLY SOLIDIFIED TO PROVIDE AN OXIDATION RESISTANT STRUCTURE WITH HIGH STRENGTH IMPARTED BY A STRONG SKELETAL-TYPE OR ALIGNED LAMELLAR CARBIDE PHASE, SOLIDIFICATION PROCEEDING IN ACCORDANCE WITH THE MONOVARIANT EUTECTIC REACTION.

Jan. 30, 1973 D. H. BOONE ET AL CAST COBALT-BASE EUTECTIG TYPE ALLOY 3 Sheets-Sheet l Filed 'April 9, 1971 Ja. 30, 1973 .Y H. BOONE ETAL 3,713,815

` CAST COBALT-BASE EUTEGTIC TYPE ALLOY Filed April 9, 1971 s sheets-sheet n Jan. 30, 1973 Q H, BOONE ET AL v 3,713,815

CAST COBALT-BASE EUTECTIC TYPE ALLOY United States Patent O CAST COBALT-BASE EUTECTIC TYPE ALLOY Donald H. Boone, North Haven, Conn., Donald A. Koss,

Houghton, Mich., and David E. Peacock, Guilford,

Conn., assignors to United Aircraft Corporation, East Hartford, Conn.

Filed Apr. 9, 1971, Ser. No. 132,836 Int. Ci. (122e 19/00 U.S. Cl. 75-171 3 Claims ABSCT OF THE DISCLOSURE A cobalt-base alloy containing chromium, carbon, aluminum and scandium or yttrium is unidirectionally soliditied to provide an oxidation resistant structure with high strength imparted by a strong skeletal-type or aligned lamellar carbide phase, solidication proceeding in accordance with the monovariant eutectic reaction.

BACKGROUND OF THE INVENTION The present invention relates to, in general, the cobalt/ chromium/carbon alloys at substantially the monovariant eutectic composition and, in particular, to such alloys provided with high temperature oxidation-erosion resistance and as unidirectionally cast.

The advantages of using controlled or ordered microstructures for obtaining high temperature strengths in various alloy systems have been known to metallurgists for sometime. One approach having particular promise is the use of eutectic castings with ordered microstructures resultant from the controlled unidirectional solidiication of certain eutectic alloys according to the techniques disclosed by Kraft 3,124,452, this patent sharing a common assignee with the present invention.

Of recent particular interest in gas turbine engine applications have been the alloy systems based on a chromium carbide reinforcement of cobalt/chromium solid solution matrices. One alloy system of monovariant eutectic composition at a nominal chemistry, by weight, comprising 49` percent chromium, 1.9 percent carbon, balance essentially cobalt, has been unidirectionally soliditied to provide about 43 volume percent of a skeletallike MzgCe-type carbide, (Cr,C`o)23C6, in a cobalt/chromium solid solution matrix. This alloy, designated UARL 236C, is more fully described in the patent to Lemkey et al. 3,552,953. Another carbide reinforced cobalt/chromium/carbon alloy solidifying according to the monovariant eutectic reaction, at a nominal composition, by weight, of 41 percent chromium, 2.4 percent carbon, balance cobalt, known as UARL 73C, may be unidirectionally solidified to provide castings incorporating aligned lamellae of a M7C3-type carbide, (Cr,Co)qC3, embedded in a cobalt/chromium solid solution matrix, as described in the patent to Thompson et al. 3,564,940. These two patents both share a common assignee with the present invention.

The cobalt/ chromium/ carbon alloy systems have dernonstrated considerable promise as strong high temperature alloys, particularly as undirectionally solidified, and considerable development has been undertaken to further` improvevtheir oxidation resistance in the very hostile dynamic high temperature oxidizing environments. Testing has shown that alloys which are dependent on ice the chromium oxides for oxidation protection are not satisfactory above 1800iD F. because of severe degradation of the underlying substrate.

Fortunately as indicated in the above patents, these alloy systems are reasonably tolerant to the addition of certain other elements to the basic chemistry in the sense that certain additions do not unduly interfere with the basic phenomenon of solidication according to the monovariant eutectic reaction upon which the principal advantageous properties of the basic alloy is dependent. 'And this tolerance may be utilized to advantage to further improve the oxidation resistance of the basic alloy per se as well as its coatability with the well-known protective aluminide coatings, particularly for very high ternperature exposure.

In connection with the coatability aspects of the basic alloy system, preliminary coatability studies of the unmodified chemistry were discouraging, partly because of the basic structure of the cast articles, i.e., the presence of large carbides, and partly because of the recognized inability to provide thick protective aluminide coatings on the cobalt-base superalloys of high strength.

SUMMARY OF THE INVENTION The present invention contemplates a high strength cobalt-base alloy containing chromium, carbon, aluminum and an active ingredient such as yttrium, scandium and the rare earth elements, substantially solidifying according to the monovariant eutectic reaction.

It contemplates a casting wherein high strength is provided by a strong skeletal-type or lamellar carbide phase embedded in a cobalt-base alloy matrix, characterized by high temperature oxidation resistance per se and/or receptibility to the generation of protective aluminide coatings.

The alloy compositions generically contemplated are those wherein the cobalt, chromium and carbon content are fixed to provide solidiication according to the monovariant eutectic reaction, particularly within the general range of 35-53 percent chromium and 1.8-2.6 percent carbon, together with about 4-8 percent aluminum, and up to about 0.5 percent yttrium, scandium or a rare earth element, balance cobalt.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photomicrograph of the alloy, Co-49Cr- 19C, illustrating the microstructure thereof comprising about 43 volume percent of a skeletal-like MZSCS carbide (Cr,Co)23Cs, in a cobalt solid solution matrix (1000x before reduction) (unetched).

FIG. 2 is a photomicrograph of an alloy of the composition Co-41Cr-2.4C showing an M703 carbide embedded in a cobalt/chromium solid solution matrix. This section was taken normal to the surface following exposure at 2012 F. for 20 hours in 0.1 atmosphere oxygen (500x before reduction) (unetched).

FIG. 3 is a photomicrograph of a transverse section of directionally solidified Co-45Cr-5.5Al-1.7C0.1Sc (1000x before reduction) (etched).

FIG. 4 shows the microstructure of a section normal to the surface of a Co46.4Cr-6Al-2C alloy exposed at 20l2 F. for 24 hours in stagnant air (1000x before reduction) (etched).

3 FIG. 5 is a graph summarizing the results of oxidation-erosion testing of the Co-46.5Cr6.2Al-2.lC-0.1Sc alloy at 2l00 F. together with coated and uncoated cornmercially available contemporary alloys.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The common and distinguishing feature of the alloys of the present invention within the ranges indicated is that, at a fixed pressure, they are basically monovariant thermodynamically and involve, with respect to the basic cobalt/ chromium/ carbon ternary system, the three phase equilibrium between the melt and two solids over a temperature and composition range and not, as in the binary or pseudo-binary systems, at a fixed temperature and composition. The monovariant ternary reaction is also to be distinguished from the ternary eutectic reaction of the type L=vc++v In the basic ternary system, those alloys of the composition, by weight, of 45-55 percent chromium, 1.7-2.2 percent carbon, balance cobalt may be solidified to form a skeletal dispersion of MZSCS type carbides in a cobaltbase matrix. Within this range the composition containing 49 percent chromium, 1.9 percent carbon, balance cobalt, known as UARL 236C, contains about 43 volume percent of the (Cr,Co)23C6 carbide. The microstructure of this alloy is shown in FIG. 1.

Those alloys of the basic composition 35-45 percent chromium, 2.2-2.6 percent carbon, balance cobalt may be unidirectionally solidied in a cobalt/chromium matrix. The alloy containing 41 percent chromium, 2.4 percent carbon balance cobalt is identified as UARL 73C.

Both of the above alloy systems have been demonstrated to be tolerant of the addition of certain other elements to the basic ternary composition in the sense that the additions do not interfere with the basic solidiiicati'on phenomena according to the monovariant eutectic reaction. This tolerance, as indicated in the patents directed to these alloy systems, is utilized to advantage herein.

In terms of mechanical properties the basic ternary alloys exhibit usable strengths to very high temperatures. However, testing has proven that cobalt alloys which depend upon the formation of Cr203 for protection from corrosive attack cannot be utilized above about l800 F. because of severe degradation of the underlying substrate. For example, metallographic examination of the UARL 73C alloy exposed to a high temperature oxidizing environment shows evidence of surface chromium depletion and internal oxidation of the carbide particles. See FIG. 2. Furthermore, although coatings comprising protective aluminides are known to provide oxidation-erosion protection to temperatures in excess of 1800* F., coatability of the basic ternary alloys is in fact limited, particularly insofar as the generation of thick aluminide coatings is concerned.

The alloys ofthe present invention obtain their inherent oxidation-erosion resistance at temperatures above 1800 F. from the preferential formation upon oxidation not of chromium oxides but of alumina while at the same time retaining the basic metallurgical structure associated with the carbide reinforced cobalt-base matrix.

An alloy of the nominal composition 49 percent chromium, 6 percent aluminum, 2 percent carbon, 0.1 percent scandium, balance cobalt was prepared and tested in oxidation-erosion at 2100 F. The results were compared with a standard cobalt-base alloy (WI-52) and a nickelbase alloy (U-700) in both an uncoated and aluminide coated condition. This alloy was observed to oxidize at a rate considerably lower than those for the uncoated WI-S 2 and U-700 alloys and similar to those for the coated WI-52 and U- 700 alloys to a point where coating failure was observed. Beyond this point, the coated conventional alloy specimens oxidize at the increased rate associated with the uncoated alloys while the alloy of the present invention continues to oxidize at the uniform and pre- 4 dictable rate. This performance is shown graphically in FIG. 5 for the alloy consisting of, by weight, 46.5 percent chromium, 6.2 percent aluminum, 2.1 percent carbon, 0.1 percent scandium, balance cobalt. The microstructure of the directionally solidified alloy comprising 45 percent chromium, 5.5 percent aluminum, 1.7 percent carbon, 0.1 percent scandium, balance cobalt is shown in FIG. 3. This may be compared with the microstructure of the alloy comprising 46.4 percent chromium, 6 percent aluminum, 2 percent carbon showing the condition of the alloy after oxidation, FIG. 4.

The WI-52 will be recognized as that having the nominal composition, by weight, of 21 percent chromium, 11 percent tungsten, 2 percent columbium plus tantalum, 1.75 percent iron, .45 percent carbon, balance cobalt. The U-700 alloy consists of, 15 percent chromium, 15.3 percent cobalt, 3.4 percent titanium, 4.3 percent aluminum, 4.4 percent molybdenum, .07 percent carbon, .02 percent boron, balance nickel.

In addition to demonstrating the oxidation resistance of these alloys, testing was also undertaken to determine their amendability to the primary and secondary fabrication techniques employed for superalloys and eutectic systems. An ingot formulated to a chemistry of 45 percent chromium, 5.5 percent aluminum, 1.7 percent carbon, 0.1 percent scandium, prepared by directional solidication techniques employed to cast other superalloys in ingot and turbine airfoil forms, readily responded to the necessary machining.

To demonstrate the mechanical property characteristics of these alloys, a mechanical property evaluation of the above alloy was also undertaken. These results are summarized in the following table. It is evident that significant loads were sustained and meaningful lives and large ductilities were obtained demonstrating the utility of this alloy as a high temperature oxidation resistant structural material. The ductility is such that even further increases in strength are possible Without reducing ductility to an unacceptable level.

While the present invention has been described in connection with certain examples and preferred process compositions and parameters, these will be understood to be illustrative only and numerous modifications will be evident to those skilled in the art from the detailed description. The invention in its broader aspects is not limited to the specific details shown and described but departures may be made from such details without departing from the principles of the invention and without sacricing its chief advantages.

What is claimed is:

1. An alloy article consisting essentially of, by weight, 35-53 percent chromium, 1.8-2.6 percent carbon, 4-8 percent aluminum, up to about 0.5 percent selected from the group consisting of yttrium, scandium and the rare earth elements, balance cobalt, unidirectionally solidied substantially according to the monovariant eutectic reaction L=a+ where or is a cobalt-chromium-aluminum base alloy and is a reinforcing carbide phase uniformly dispersed in a consisting essentially of a carbide selected from the group consisting of (Cr,C0)23C6 and (Cr,Co)-,C3.

2. A unidirectionally solidified alloy article consisting essentially of, by weight, 45-55 percent chromium, 1.7- 2.2 percent carbon, 4-8 percent aluminum, up to about 0.5 percent yttrium, scandium or a rare earth element, balance essentially cobalt, said alloy having a microstructure comprising a dispersion of (Cr,Co)23C6 type carbides in a cobalt-base alloy matrix of high chromium and alu- References Cited minum content.

3. A unidirectionally solidified alloy article consisting UNITED STATES PATENTS essentially of, by Weight, 35-45 percent chromium, 2.2- 3552953 1/1971 Lemkey et al- 75-171 2.6 percent carbon, 4-8 percent aluminum, up to about 5 3564940 2/1971 Thompson et al '75*-171 0.5 percent yttrium, scandium or a rare earth element, balance essentially cobalt, said alloy having a microstruc- RICHARD o DEAN Prlmary Exammer ture comprising a dispersion of (Cr,C0)7C3 type carbides U S C] X R in substantial alignment in a cobalt-base alloy matrix of high chromium and aluminum content. 10 75-134 F, 176; 148-32 

